It is generally known in the welding art to automatically supply a band of filler material, typically a wire of filler metal (or "filler wire"), to the vicinity of an electric arc created between a consumable or non-consumable electrode and a workpiece. In some cases the wire of filler material is the electrode; in others, a separate electrode is used. It is important that the various weld parameters, such as the current and voltage of the arc and the rate of supply of the filler material, be adjustable by the operator to ensure proper welding. Additionally, in many applications the workpiece is automatically moved relative to the electrode and the supply of filler material.
Welding systems have also been developed in which the rate at which the filler material is delivered to the vicinity of the arc is varied, to obtain various improvements in the welding process and in the versatility of use of the particular welding equipment. See, for example, U.S. Pat. No. 4,159,410 to Cooper. The Cooper patent shows several embodiments of devices for varying the rate at which the filler material is supplied to the vicinity of the arc.
In a first embodiment, the torch assembly (that is, the electrode and its supporting structure and a wire guide for guiding the filler wire into the vicinity of the arc) is moved in a reciprocating manner with respect to the workpiece. The welding machine also includes filler wire feed drive rolls which are cyclically rotated in forward and reverse directions, with the net forward rotation exceeding the reverse rotation, so that a net supply of wire to the vicinity of the arc occurs.
In another embodiment, the feed rolls rotate continuously while supplying the wire to a flexible wireguide conduit carrying the wire to the vicinity of the arc. The end of the conduit is reciprocated with respect to the electrode, such that the rate at which the wire is actually supplied to the arc varies. The Cooper patent also teaches synchronization of variation of the weld current with the motion of the filler wire with respect to the arc. See column 5, lines 54-63 of the Cooper patent.
U.S. Pat. No. 4,019,016 to Friedman et al. which is commonly assigned with the present application, discloses a welding control system in which the position of the torch assembly is moved in an oscillatory fashion back and forth with respect to the center line of a weld to be made. For example, if two pipes are to be butt-jointed, the torch is oscillated back and forth longitudinally across the joint, while the overall assembly moves circumferentially around the joint at a slower rate, such that the filler metal is deposited in a zig-zag pattern across the joint to be formed.
The Friedman et al patent teaches that the weld current may be varied in synchronization with the movement of the torch back and forth across the joint. In this way it can be ensured that the weld current (and hence the heat applied) corresponds to the cross-sectional area of the tubes to be joined, such that the appropriate amount of heat is supplied thereto. The Friedman et al patent also teaches automatic control of the wire feed rate in accordance with the position of the torch assembly with respect to the joint, and suggests feeding the filler wire at rates proportional to the current being supplied to the electrode. See column 5, lines 31-45; column 10, lines 22-59.
U.S. Pat. No. 4,441,012 to Risbeck et al teaches a further improvement in a device for automatically controlling the welding current and the rate of supply of filler metal to a welding arc. According to this patent, a particular advantage is obtained when the weld current and the wire feed rate are pulsed such that the weld current is at its maximum just as the tip of the filler wire touches the molten pool formed by the arc.